This invention relates generally to indicators used in industrial process control systems. More particularly, the present invention relates to process transmitters having temperature responsive indicators.
Process transmitters are used to remotely or locally monitor process variables, such as pressure, temperature, flow and level, of process fluids used in industrial processes. Process transmitters include sensors or transducers that produce an electrical output in response to physical changes in the process variable. For example, capacitive pressure transducers or piezoresistive pressure transducers produce an electrical signal as a function of the pressure of a process fluid. The electrical signal of the sensor is processed by the transmitter circuitry to produce an electrical output that can be monitored as an indication of pressure of the process fluid. Process transmitters also include electronics for either remotely or locally monitoring the electrical output. Remotely monitored transmitters include electronics that transmit the electrical output over a control loop or network to a central monitoring location such as a control room. Locally monitored transmitters include displays, such as digital LCD screens or analog dials, that exhibit the electrical output at the site of the process transmitter in a readable format. In other embodiments, process transmitters include components for both local and remote monitoring.
Digital and analog displays can provide a highly precise indication of the measured process variable such that very small changes in the measured process variable are easily detected. However, such precise measurements are not always required. For certain applications, digital and analog displays provide unneeded accuracy and are therefore more expensive than necessary for such applications. Sometimes, only a quick visual indication of the approximate magnitude of the process variable reading is necessary. For instance, maintenance personnel conducting routine checks of production facilities may only need to know if process transmitters are operating and the sensed process variable is at a safe level. This is particularly so when the precise process variable reading is simultaneously being transmitted to and monitored in the control room. Thus, there is a need for an accurate, low-cost indicator for process transmitter displays.
Power consumption is another drawback associated with digital and analog displays. Process transmitters typically draw their power through the control loop or network, or through wiring systems typically found in industrial settings. In order to ensure compatibility with particular control loops or networks, process transmitters are often required to draw less than 4 mA of current. As advances in micro-technology and data transmission progress, it continues to be a challenge to design electronics that are able to operate below a current draw of 4 mA and provide advanced data transmission. Having the additional burden of powering a display unit provides an additional design challenge. Thus, there is a need for process transmitter indicators that have low power consumption.
Also, process transmitters are designed to function within defined temperature ranges. Typically the ambient operating temperature limit of transmitters is −40° F. (−40° C.) to 185° F. (85° C.) and the storage temperature limit is −50° F. (−51° C.) to 230° F. (110° C.). Thus there is a need for a process transmitter having a low cost visual indication when these temperatures are exceeded.